The operation targeted people sharing "the most extreme forms of video material," including images of babies and toddlers being sexually abused or raped, the European police coordination agency said.
There are 269 suspects so far and more arrests are likely after the operation uncovered previously unknown networks of child sex offenders operating on different Internet chat channels, Europol director Rob Wainwright.
He called the operation a success, but noted in a statement it shows "how the Internet is helping offenders to develop better techniques for sharing images on a global basis and for protecting their identity."
"The problems involved are becoming harder to police," he said.
Among other issues, the sheer volume of encrypted material is daunting, and much of the information seized in raids is still awaiting analysis. A single suspect in Switzerland had more than 120 terabytes of data - amounting to thousands of hours of high-definition video footage.
The investigation, code named "Operation Icarus," was carried out under the leadership of Danish police, due to Danish expertise in analyzing the peer-to-peer networks that were used to share files. Nineteen men aged 24 to 55 years old are facing preliminary charges in Denmark.
Denmark's chief of national police, Jens Henrik Hoejbjerg, said 59 computers and 2,430 storage devices had been seized in Denmark alone.
"It's a huge amount for our investigators to handle," said Hoejbjerg.
Europol has its headquarters in The Hague, Netherlands.
The nations involved in the investigation were Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Ireland, Italy, Luxembourg, Malta, Netherlands, Poland, Slovakia, Spain, Sweden, Croatia, Norway and Switzerland.
Investigators said one of the men arrested was in the process of grooming a young child and was arrested before an attempted face-to-face meeting. They did not disclose where.
EU Home Affairs Commissioner Cecilia Malmstrom said the operation showed the "importance of cooperation between law enforcement authorities at European and international level to tackle criminal activities that know no borders."